HashSet and TreeMap

[Raj Ohadi]

1. when we do TreeMap's remove(obj), does that remove

a) both key (obj) and value that maps to the key
or
b) only the value that maps to the key

??

2. If I have a HashSert set_1 and HashSet set_2, how can I find the objects that are in set_2 but not in set_1, i.e. set_2 - set_1 ?
I don't find such method.

thanks

[Rob Spoor]

The mapping is removed. This means both key and value. If you only want to remove the value, use "put(key, null)".

The second one can be obtained using addAll and removeAll:

HashSet difference = new HashSet();
difference.addAll(set_1);
difference.removeAll(set_2);

With most collections you can combine the first two lines:

HashSet difference = new HashSet(set_1);

That second answer is in the API of Set#removeAll though:

Removes from this set all of its elements that are contained in the specified collection (optional operation). If the specified collection is also a set, this operation effectively modifies this set so that its value is the asymmetric set difference of the two sets.

[ October 02, 2008: Message edited by: Rob Prime ]